1. Field of the Invention
The invention relates to the use of superconducting magnets in energy storage systems. More particularly, the invention relates to the use of a modular superconducting magnet design to provide a simple and effective energy storage device having a capacity that can be upgraded by the addition of supplemental superconducting magnet modules.
2. Description of the Prior Art
Superconducting magnets were first developed to provide extremely large magnetic fields for use primarily in large scale particle accelerators. Superconducting magnets are also known to have application in the storage of substantial amounts of energy for prolonged periods of time. Although the benefits of using superconducting magnets in such applications are well known, and several systems have even been suggested that would utilize these benefits, many difficulties are still encountered that diminish the advantages of using superconducting magnets in commercial applications. One application for which such magnets might otherwise be of substantial benefit is in smoothing out variations in electricity demand over a twenty-four hour period. See, for example, Application Ser. No. 385,104, now U.S. Pat. No. 4,962,345, entitled "Superconducting Voltage Stabilizer" filed concurrently herewith.
The difficulties encountered with conventional superconducting magnets result to some extent from the design of these superconducting magnets and the techniques and materials consequently adopted for their manufacture. Typically, a superconducting magnet is wound in a manner similar to a spool of thread, from a single length of superconducting cable. The windings form a solenoid having several layers, with adjacent turns in each layer being disposed in axial alignment.
The magnetic fields produced by the adjacent loops of a solenoid add to give a field parallel to its axis. To maximize the magnetic field, it is important to ensure that current flows around the loops of the solenoid rather than parallel to its axis. The windings of the coil must therefore be insulated one from another.
The energy stored in any solenoid is proportional to the square of the magnetic field produced. In the case of a uniform field the stored energy is: ##EQU1##
where
E is energy in joules; PA1 B is magnetic field in Tesla; and PA1 .mu..sub.o is the magnetic permeability of free space. PA1 V.sub.L =L.times.dI/dt PA1 V.sub.L is the back voltage across the inductor; PA1 L is the characteristic inductance of the device; and PA1 dI/dt is the instantaneous change in current through the device.
Thus, to maximize the amount of stored energy for a given current, the magnetic field produced by that current must also be maximized. To achieve this, the conductors must either be placed as closely together as possible or a core material of high magnetic permeability should be used, preferably, a combination of both.
Solenoids and other wound devices are inductive, however, and when subjected to a varying current, a voltage opposing the change in current is induced. The magnitude of the voltage obeys the following relation:
Where
Devices with large inductances, or that experience large variations in current, will therefore experience high back voltages. In the case of large superconducting magnets wound like a spool of thread, the induced voltage between adjacent conductors may be extremely high. As the ability of an insulator to withstand voltage is directly related to its thickness, the insulation around the coil windings in such large solenoids must be very thick. This thick insulation increases the distance between adjacent windings and correspondingly reduces the coupling effect between them. As it is vital that the insulation around the superconducting cables does not break down when subjected to large changes in current, high power superconducting magnets tend to be larger, have more turns, and be less efficient than a simple extrapolation of a smaller superconducting magnet would predict.
It would be desirable, therefore, to provide a superconducting magnet in which coupling between adjacent conductors is strong in order to create a more efficient energy storage unit.
The manufacture of large superconducting magnets is complicated by the tendency for conventional magnets to be wound from a single length of superconducting cable regardless of its capacity. Long lengths of superconducting cable of the necessary quality can be difficult to obtain and are inclined to be expensive. Waste of superconducting cable often occurs because there is insufficient length to wind the entire magnet, or because damage occurs to the cable during winding. This can add considerably to the cost of manufacturing the magnets further deterring their commercial utilization. It would be desirable, therefore, to provide a superconducting magnet that substantially reduces the wastage of superconducting cable in the manufacture of a large capacity magnet, and to thereby provide a lower cost alternative to conventional superconducting magnets.
Conventional superconducting magnets additionally tend to be inflexible in their storage capacity. As the length of the conductor determines the capacity of the magnet, the only practical way to increase the capacity of a storage system is to replace the existing superconducting magnet with one that is entirely new and larger. In applications where the storage capacity of a system may desirably be upgraded on a relatively frequent basis, the utilization of a conventional superconducting magnet as a storage unit is not an attractive option. It would therefore be advantageous to provide a superconducting magnet that could be upgraded without requiring an entirely new magnet to be wound and substituted for the existing magnet.
Another factor that influences the cost and therefore the commercial viability of a superconducting magnet is the probability that the final product will be of adequate quality. For all wound magnets, the only way the quality of the magnet can be verified is by inspecting the parts prior to winding, and testing the magnet afterward. Any damage in the interim is difficult if not impossible to detect. As winding a large conventional superconducting magnet is likely to take a significant length of time, it would be useful to provide a large capacity magnet for which the time taken in winding a defective coil is reduced, enabling manufacturing costs to be correspondingly decreased.
It would be desirable, therefore, to provide a superconducting magnet that has adequate storage capacity and can easily be upgraded to meet increased storage requirements. It would also be desirable for the magnet to be designed such that wastage of both material and manufacturing time is reduced. The provision of such a superconducting magnet would greatly enhance the commercial appeal of superconducting magnets as energy storage units.
The modular design of the present invention provides such a superconducting magnet. The magnet is built from modular units connected in series to provide the desired storage capacity. This arrangement enables additional units to be added to upgrade the magnet's capacity as desired, and in addition, as each individual unit is smaller than the final magnet, problems associated with scale are avoided. Quality assurance is also enhanced because individual units can be verified prior to assembly, and only those units found satisfactory joined to form large modular superconductors.